A carbon monoxide detector, or CO detector, sounds an alarm to prevent poisoning by carbon monoxide (CO), a colorless and odorless compound produced by incomplete combustion that is lethal at concentrations of 100 parts per million (ppm). OSHA limits long-term workplace exposure levels to 50 ppm. In a residence, CO can be formed by open flames, space heaters, water heaters, blocked chimneys or running a car inside a garage.